Shock absorbers are typically oil-filled cylinders within which a vented piston is mounted. The piston is connected to a shaft which extends out of one end of the cylinder. The outer end of the shaft is mounted to one point on the vehicle and the other end of the cylinder is mounted to another point on the vehicle, sometimes in parallel with a suspension spring.
Typical shock absorbers provide two kinds of damping: compression damping and rebound damping. One refers to damping force created during “inward” travel of the shaft (shortening of the shock), the other refers to force created during “outward” travel of the shaft (lengthening of the shock). Generally, but not always—depending on the linkage connecting the shock absorber to the vehicle, rebound damping occurs during outward motion and compression damping occurs during inward motion.
Piston-type shock absorbers can be designed to provide the same amount of damping on both the compression stroke and the rebound stroke. Alternatively, the fluid passageways through the vented piston can be designed so that the restriction to fluid flow through the vented piston during the compression stroke is different than the restriction to fluid flow during the rebound stroke. In this case the damping during the entire compression stroke is different than the damping during the entire rebound stroke. The fluid passageways through the vented piston are covered by circular plates, or discs which restrict and/or prevent the flow of fluid through the passageways to obtain the necessary compression and rebound characteristics needed for the particular application. For a constant velocity, the resistance provided by the working piston is constant for the entire stroke of the vented piston.
In some applications where the vehicle traverses uneven terrain, it is desirable to have different damping characteristics at different places along the stroke of the piston. For example, at the beginning of the stroke, starting from fully extended, it might be desirable to have very soft damping, thus absorbing impact as much as possible, but near the end of the stroke, it is desirable to have a very hard damping to prevent the shock from bottoming, being fully compressed to the point that there is a metal to metal contact between the moving parts. With many standard shock absorbers, this is not possible since the valving is non-adjustable and only speed sensitive.
Another type of damping is called position-sensitive damping. Position-sensitive damping is typically achieved by the combination of conventional vented piston damping, with the oil flowing through the piston, plus the passage of oil around the piston through a by-pass chamber or channel, which permits oil to by-pass the piston during a portion of the piston stroke. These shock absorbers are known as twin-tube shock absorbers. The by-pass channel thus permits lesser damping over the portion of the stroke during which some fluid flows around the piston through the by-pass channel. Therefore, the shock absorber can have different damping characteristics along different segments of the stroke. A single shock absorber can provide smooth damping for less aggressive riding and firm damping for aggressive riding without making any adjustments during the ride. For example, the shocks can provide reduced damping in the mid-stroke zone, where the shock is most active while trail riding or other less aggressive riding. If the rider starts riding more aggressively, or hits a large bump, causing the shock absorber to compress deeper into the stroke, the piston extends beyond the by-pass passages and again the shock absorber relies on the conventional piston damping. This type of shock absorber has been available for many years. U.S. Pat. No. 5,178,239 and U.S. Pat. No. 6,296,092 illustrate examples of position-sensitive damping action via by-pass channels.
However, position-sensitive shock absorbers using twin tubes to create by-pass channels are bulkier than standard shock absorbers and require an intricate series of by-pass channels linked to the cylinders.
Thus, there is a need for a position-sensitive shock absorber that alleviates some of the drawbacks of prior art position-sensitive shock absorbers.